consistent with less reverse migration, due to the slow rate of drying. The
strength of beams depends more on the upper and lower surface layers
than on the core; therefore, a greater concentration of consolidant in the
surface layers would tend to increase bending strength of the samples.Among the seven criteria for selection of consolidants discussed above, the
most important positive characteristic is that the consolidant serve as an
effective strengthener. This characteristic depends on a number of factors,
including the amount and distribution ofthe consolidant, as well as the
properties of the solidified consolidant itself.Properties of wood and consolidant composites
The addition of consolidant to deteriorated wood produces a composite
with resultant properties that depend on the relative amounts and proper-
ties of both components involved. One method for predicting the mechani-
cal properties of composites is the so-called rule of mixtures, which can be
stated as follows for the modulus of elasticity (Siau et al. 1968):E 5 EwVw 1 EpVp 1 EaVa (5a)where: E 5 modulus of elasticity of the composite;Ew, Ep, Ea 5 modulus
of elasticity of wood substance, consolidant, and air, respectively; and
Vw,Vp,Va 5 volume fraction of wood substance, consolidant, and air,
respectively.
If we remove the term for the consolidant from the right-hand
side of Equation 5a, we get the modulus of elasticity of gross wood, Eg,
asa composite of wood substance and air. Since the consolidant is simply
contained in the air space of the porous wood, Equation 5a can therefore
also be given as (Wang and Schniewind 1985):E 5 EpVp 1 Eg (5b)This equation can also be used for estimating other mechanical
properties of wood and consolidant composites. Wang and Schniewind used
Equation 5b to estimate both strength and stiffness in bending of treated
Douglas-fir samples, and they obtained reasonably good agreement with
actual test results. However, estimates tended to be on the low side—the
probable reason being a greater concentration of consolidant in the surface
layers, which would tend to improve the bending strength of the composite
more than a uniform distribution (Schniewind and Eastman 1994).
Since the volume fraction of consolidant, Vp,is one of the factors
in Equation 5b, it follows that the strengthening effect of consolidation
should be positively related to the amount of loading achieved. This is not
only intuitively obvious but also shown to be true by experimental results
for monomers polymerized in situ (Simunkovà, Smejkalovà, and Zelinger
1983) and for polymers introduced in solution (Wang and Schniewind
1985). The value ofVpin Equation 5b will always be less than 1. The con-
tribution of the consolidant to the properties of the composite will there-
fore depend highly on the strength of the consolidant in relation to the
strength of the wood. As an extreme example, a polymer that has 10% of
the strength ofnormal wood, if impregnated to a volume fraction of 10%,Consolidant Effectiveness
96 Schniewind